Measured versus calculated net radiation and soil surface heat flux values in an automated weather station in arid lands, and its application in the FAO Penman-Monteith method

For efficient irrigation timing in arid land agriculture, it is essential to know the daily evapotranspiration rate of each crop (ETc) established in a specific location. ETc can be obtained from the reference evapotranspiration rate (ETr) according to the procedure of the FAO-56 method and the crop coefficient development (Kc). However, the daily rates of ETr obtained with calculated values of net radiation (Rn) and soil surface heat flux (G) may differ from those obtained when Rn and G are measured in the same automated weather station. The objective of this study was to evaluate the difference between the daily rate of ETr obtained with the FAO-56 method using calculated and measured data of net radiation and soil surface heat flux in an arid land. On an arid land of northern Mexico, a Campbell-brand automated weather station (Campell Sci., Inc. Logan, Utah, USA) was located at the center of a circular area of 12 m in diameter, with a green grass of full surface coverage 12 cm height with no soil moisture deficit, to measure at a height of 2 m (1 s scan and 30 min averaged values) the air temperature and relative humidity, wind speed and direction, incident solar radiation, barometric pressure, and rainfall. Furthermore, the net radiation over the grass and the soil surface heat flux were also measured. The data generated by the automated weather station was used to calculate the daily rate of ETr (FAO-56 method) using both calculated and measured values of net radiation and soil surface heat flux. The results of this study showed that the ETr (FAO-56 method) obtained with Rn and G measured was 17.63% higher than the ETr (FAO-56 method) determined with Rn and G calculated. The Rn measured (using a net radiometer) was 14.08% larger than the Rn calculated (FAO-56 method). G measured was 36.6% smaller than G calculated as 10% of the Rn (FAO-56 method). The daily ETr rate (FAO-56 method) using Rn and G measured in the automated weather station was higher than the ETr rate (FAO-56 method) obtained with Rn and G calculated, due to a bigger value of Rn and a lower value of G measured in the automated weather station than the values of Rn and G calculated by the FAO-56 method.